It must be very low probability. I have had an Archmage casting Wonder every turn for more than 100 turns and no Baboon as of yet. I get 3-5 random effects each time I cast, and no Baboon! Luckily, there are other trophies to achieve this game (I have a Flesh Golem up to 13 str and a host of Mimics getting ready to invade the other continent in hopes of getting 20+ promotions) so it is not an endless click fest of end turn trying to get the Baboon. I will be disappointed if it doesn't come before the end of this game though.